For material handling equipment which is supported and driven by endless crawler drive tracks, the length of the ground-engaging portion of the lower run of the crawler tracks has a significant effect on the performance characteristics of the equipment. Typically, the maximum length of the ground-engaging portion of a crawler track generally corresponds to the distance between an idler roller and a drive sprocket about which the endless track extends. In many situations, it is desirable to have this maximum length of the track in engagement with the ground. Maximum ground contact desirably provides necessary support and traction for a piece of equipment (and relatively low per square inch ground pressure) in soft soil conditions. The usual track support arrangement, including the typical rigid mounting of the track-engaging rollers of the crawler undercarriage, desirably provides relatively good stability for the machine supported by the crawler tracks.
In contrast, maximum track engagement with the ground can be undesirable under some circumstances, particularly where mobility is needed. Based on available horsepower, relatively low travel speed and high bar pull are necessary to make turns when track contact is relatively large (draw bar pull being an essentially linear function of track contact area). This can be undesirable when relatively high travel speed is needed, such as for driving between different areas of a job site. Additionally, maximum speed is also somewhat restricted because the typical crawler track mounting arrangement is rigid and provides no cushioning or shock absorption for either the machine, or the person operating it. Further, the typical crawler track mounting arrangement is not particularly well suited for traveling over small objects or obstacles at relatively high travel speeds with lower draw bar pull.
In view of the above, an arrangement for selectively altering the length of the ground-engaging portion of a crawler track is very desirable, and enhances the versatility and efficiency of material handling equipment. By permitting a relatively shorter length of an implement's crawler tracks to be in contact with the ground, relatively less draw bar pull is required for turning, thus enhancing maneuverability. Further, the requirement of less draw bar pull permits the available horsepower of an implement to be used for relatively higher travel speeds. Additionally, improved shock absorption can be provided if the arrangement for altering the ground-engaging length of a crawler track is provided with energy absorption capability. Finally, a more favorable angle of approach with respect to obstacles can be obtained, thereby permitting travel over objects with less draw bar pull, and thus higher travel speeds.
While the virtues of an arrangement for changing the portion of a crawler track which contacts the ground are apparent, such an arrangement must be straightforward and reliable in construction if the virtues are to outweigh problems of excessive maintenance and installation expense. Because material handling equipment is ordinarily subjected to harsh operating conditions, components of the equipment must operate reliably without excessive service requirements. At the same time, reliability cannot be obtained at the expense of economic fabrication if an arrangement is to be commercially viable.
Arrangements are known for altering the ground-engaging portion of a crawler track, yet none have been widely commercially employed, particularly in material handling equipment. For example, U.S. Pat. No. 2,860,715, to Bouffort, illustrates an adjustable arrangement for endless tracks which is relatively complicated, and which has not found ready commercial acceptance. Additionally, the arrangement illustrated in this patent is not particularly suited for use in material handling equipment since the arrangement does not permit the entire length of the lower run of a crawler track to engage the ground. As noted above, maximum ground contact is necessary for a crawler-supported material handling device to properly and efficiently perform material handling functions.
Because of the advantages which selective alteration of the length of the ground-engaging portion of a crawler track provides, it is desirable to provide an arrangement for this purpose so long as it is straightforward in construction for reliability and ease of manufacture. Additional advantages can be obtained with such an arrangement if track shock absorption is provided. The provision of such an arrangement for material handling equipment desirably enhances efficiency and cost-effectiveness of material handling operations.